How did antennas get so small?

Antennas are among the most “analog” of all components, functioning as bidirectional transducers between ambient RF energy and electrical current flow. For many years, most antennas were physically obvious and visual, ranging from modest whip antennas (formally called a monopole or Marconi) with ground planes used on portable and car radios, to the big dishes used for radio astronomy or radar (Figure 1).

Yes, it’s called an “aerial” in some countries, but we’ll just stick with “antenna” here.

Figure 1 The antenna “family tree” is complicated and somewhat bewildering; this is just one possible version. Source: Pressbooks

Other installations use some variation of the dipole antenna, such as the classic TV “rabbit ears” from the 1950s and 1960s (Figure 2). Regardless of type, antennas made a clear statement: they were tangible, and they were doing critical wireless work.

Figure 2 This dipole antenna, affectionally called a “rabbit ears” antenna, was a standard set-top fixture on analog VHF TVs (the loop antenna is for the added UHF band). Source: Wideskall via Wal-Mart

Now, antennas are often invisible even on products which used to have visible ones. Consider the cell phone and smartphone. I was doing some research into the history of cell phones since the first commercial units—when they were just phones and not “smart” —and one thing struck me: they all had external antennas, about 12 inches (30 cm) long (Figure 3).

Figure 3 The all-analog Motorola DynaTAC 8000x (1983) was the first commercial cell phone. Due to its high price ($3,995 at launch), it also became a status symbol despite its one-hour talk time. Source: PC Magazine

Some were short whips, some were so-called “rubber duckies” where the antenna was wound around a semi-ridged post. For some cell phones, such as the classic Motorola Star-Tac flip phone, the unit would fit in the pocket, but the user had to pull the antenna out of the phone body enclosure to use the phone (Figure 4).

Figure 4 One of the earliest successful cell phones, this Motorola MicroTAC Classic was released in 1991 and supported a single RF band via its extendable whip (monopole) antenna. Source: Southside Allstars

One Dell desktop PC I had about 15 or 20 years ago had an external detachable antenna for Wi-Fi access. While that seemed like a nuisance, it actually was a benefit as it allowed me to use a separate cable-connected antenna from D-Link in its place and locate it to get a better signal (Figure 5). Now, all the desktop PCs that I have checked feature a small permanent antenna on or just inside their case, and no ability to add an antenna (yes, there are various boosters and repeaters to solve the problem, but they are active and require setup).

Figure 5 The ANT700-2400 2.4 GHz Wi-Fi antenna, with supplied cable and connector, allowed me to easily move the antenna from the PC to a better location. Source: D-Link Australia

That was then… “now” is very different

Antenna reality has changed dramatically, even if the basic physics and Maxwell’s equations have not. Smartphones and just about any wireless-connected consumer device—phone, router, smart “whatever”—now implement connectivity with an antenna that is embedded in the unit. This simplifies packing, avoids user breakage, eliminates the need for a discrete antenna connector, and presents a sleeker, more user-friendly product. It’s almost as if a magician stepped in and made the visible antenna disappear right in front of our eyes.

How did effective antennas go from larger external add-ons to tiny internal ones? Part of the reason is the migration to higher frequencies with shorter wavelengths, but that’s only part of it. After all, going from 500 MHz to 1 GHz cuts the wavelength in half, but these internal antennas are far smaller than one-half of those earlier ones.

A large part is due to new material technologies, aided by advanced electromagnetic modeling and simulation.

First, there is the microstrip patch antenna, which uses the PCB copper itself as the antenna and surrounding ground plane. This flat, low-profile antenna is made of a metal patch on one side of the circuit board and a solid metal ground layer on the other.

It’s compact with no direct cost and can even be configured for multi-band performance. However, it occupies PCB real estate and requires careful management of its dimensions as well sufficient ground plane, and may have an unacceptable radiation pattern.

As a result, the simple microstrip antenna may not be a suitable option despite its apparent benefits. For these reasons, patch and other specialty antennas may offer the form factor and specifications needed for an internal antenna.

For example, there’s the planar inverted-F antenna (PIFA), not to be confused with the PIGA, a pendulous integrating gyroscopic accelerometer. The PIFA starts with the inverted-F antenna (IFA) —proposed in 1958—and is a variant of the patch antenna. In this arrangement, the monopole element runs parallel to a ground plane and grounded at one end, and the antenna has a low impedance on the order of a few ohms (the classic base-fed λ/4-wavelength monopole has an impedance of 36.5 Ω).

The antenna feed is placed at an intermediate point a short distance from the grounded end. By adjusting the placement of the feed and other “tweaks and trims”, its impedance can be made to match the power amplifier (PA) feed. So, it’s an efficient radiator without the need for additional matching components.

The original inverted-F antenna used a bent wire for its monopole. The PIFA modifies the IFA by using a flat element placed immediately above the ground plane with a shorting pin between them. PIFA is defined by just a few basic dimensions (Figure 6).

Figure 6 The planar inverted-F antenna (PIFA) shown above is a modified inverted-F antenna (IFA) with a flat element rather than a bent wire above the ground plane. Below are shown its critical dimensions. Sources: Springer Nature; European Union Digital Library

Another embedded option is a ceramic-chip antenna, such as the Abracon ACR4006X 600-6000 MHz wideband ceramic chip antenna, a surface-mount device measuring just 40 mm × 6 mm × 5 mm. In operation, it requires a tiny LC impedance-matching network consisting of an 8.2 nanohenry (nH) inductor and a 3.9 picofarad (pF) capacitor (each of 0402 size) to achieve the desired 50-Ω impedance (Figure 7).

Figure 7 The ACR4006X 600-6000 MHz wideband ceramic chip antenna has a footprint of just 40 mm × 6 mm and requires only two tiny passive components for 50-Ω impedance matching. Source: Abracon LLC

The ACR4006X datasheet indicates that it’s a 600 to 6000 MHz device, but notes that its efficiency, peak gain, and average gain graphs have some gaps. This is deliberate, as the multi-band antenna is designed and optimized for performance in three specific bands in that wider range: 600 to 960, 1710 to 2690, and 3300 to 6000 MHz to support 3G, 4G, and 5G allocations as well as some smaller spectrum allocations. Other interesting tiny antennas are offered by vendors such as Taoglas Group.

The incredibly shrinking antenna

Not only have wireless-related ICs themselves shrunk remarkably as their functional capabilities have increased, but a non-electronic, passive, and yet essential part of the RF signal chain—the antenna—has also shrunk toward embedded invisibility, largely due to advances in materials, modeling, and simulation.

Certainly, there are many low-power applications, especially at lower frequencies in the tens of megahertz and below, that mandate larger, external antennas. But as operating frequencies cross into the gigahertz and tens of gigahertz zone, these tiny antennas are especially viable.

But do you miss the performance flexibility of the older antennas? I do, sometimes, and maybe I also miss their tangible appearance, telling us all they have a role to play.

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

Related Content

The post How did antennas get so small? appeared first on EDN.

Become a member

Become a member

Become a subscriber

Become a sponsor